1. Field of the Invention
The present invention relates to a projection zoom lens, and more particularly to a projection zoom lens for use in a projection optical system of a projector and also use in an image-taking optical system of an image-taking device.
2. Description of the Prior Art
Projectors, such as a CRT (Cathode Ray Tube) projector, an LCD (Liquid Crystal Display) projector, a DLP (Digital Light Processing) projector, and an LCoS (Liquid Crystal on Silicon) projector, can project an image or other information on a large screen. For example, a projection optical system of the DLP projector uses a DMD (Digital Micro-mirror Device) modulator as a light valve, wherein minute mirror-surface elements (micro-mirrors) are arranged in an array corresponding to pixels and form an image by controlling the angles of the respective mirror-surfaces, and a projection zoom lens enlarges said image to be displayed on the large screen.
Quality of a projection image displayed on the large screen is generally determined by the light valve, the projection zoom lens, and other imaging components thereof. Generally, the projection zoom lens is a compound lens composed of a combination of a plurality of lenses, at least including a converging lens and a diverging lens, and is used to minimize deterioration in resolution, spherical aberration, chromatic aberration, and other type of aberrations.
A conditional projection zoom lens, which is disclosed in U.S. Pat. No. 6,590,716 B2, comprises a first lens group having a negative refractive power, a second lens group having a positive refractive power, a third lens group having a negative refractive power, and a fourth lens group having a positive refractive power. Although the conditional projection zoom lens effectively reduces aberrations and improves the quality of a projected image, it is not fit for use in projectors because an optical system employing the four-group zoom lens will be bulky, and the manufacturing cost thereof will be increased and the assembly thereof will be complicated.
U.S. Pat. No. 5,619,381 discloses a two-group movable lens, including a zoom lens group having a negative refractive power and a compensator group having a positive refractive power. Both groups contain at least one aspheric lens element for elimination of aberrations. A projection optical system employing the movable lens groups has a low distortion and high resolution with fewer lens elements. However, this patent only roughly introduces the configuration and the function of the two-group movable lens, and does not provide any detailed description of conditions and parameters necessary to the design.
Hence, it still needs to provide a projection zoom lens for resolving the question of aberrations existing in the currently available designs and devices.